BERKELEY, Calif. -- University scientists assert their study on the importance of wild bees to farming makes a case for preserving grasslands that provide a key habitat for the bees.

California agriculture reaps between $937 million and $2.4 billion a year in economic value from free-living bee species that pollinate crops, according to University of California-Berkeley researchers.

About one-third of the value of the state's agriculture comes from pollinator-dependent crops, representing a net value of $11.7 billion a year, the study found.

The research, published in the June issue of the journal Rangelands, debunks a widely held assumption that wild pollinators are not a significant source of crop pollination. Indeed, wild bees provide an estimated 35 percent to 39 percent of pollination to the state's crops.

"We were pretty surprised by that result," said Claire Kremen, an environmental science associate professor at UC-Berkeley and the senior author of the study.

Kremen said the researchers expected wild-bee pollination to occur the most on the edge of the Central Valley, where farmland is adjacent to natural habitat.

"If you get a kilometer or so away from natural habitat, you're not getting wild bees," she said. "What I attribute (the high percentage) to is the topographic diversity of the state. You have these agricultural valleys like Salinas Valley ... surrounded by a lot of incredible natural areas."

Kremen and other researchers say the study reinforces the notion that preserving rangelands has significant economic value, not only for ranchers who graze their cattle but also for nearby farmers.

The study comes as some California farmers and ranchers manage their land to benefit native pollinators, partly as a result of the 2008 Farm Bill, notes Ria de Grassi, the California Farm Bureau Federation's director of animal welfare, livestock and agricultural research policy.

The legislation included provisions for landowners to encourage native pollinators as well as managed bee colonies, she told the Capital Press in an e-mail.

"The new report appropriately points out the importance of conserving California farmland, including rangeland, because of its environmental and economic benefits," she said.

Kremen notes the use of wild pollinators can save farmers at least some of the cost of placing rented hives, and she suggests three ways that landowners can enhance their wild bee habitat.

First, riparian buffers would provide habitat for bees and other wildlife as well as flood control and water purification, she said. Secondly, building hedgerows of native plants around farm borders would promote pollinators and natural enemies to pests without taking land out of production.

"The other thing I firmly believe in, although it's more of a stretch for a lot of growers, is bringing more diversity into the crop field itself," Kremen said. "Different crops on the farm field that bloom at different times ... are less subject to pest outbreaks and require less pesticides."

She said wild bees could help take pressure off honeybees, as nearly all available colonies in the United States must now be used to pollinate California almonds. As a result, evidence shows honeybees are suffering from nutritional deficiencies, she said.

"Farmers know what they're doing," Kremen said. "The farmers who are near natural habitat don't bring bees in. ... They don't pay for something they don't need. If they're bringing them in, it's because they need them."